I just bought an iPhone 3GS and wondering what applications that can help on .NET development.
First thing I would do is start listening to Podcasts on .Net development and download them on to the iPod in the iPhone, using iTunes
Here's a link that contains links to such podcasts
If you are specifically looking for apps from appstore, then I don't know, may be search?
And, if you are looking to develop apps for iPhone in .Net, then yeah it's possible using Mono AOT(Ahead of Time)Compilation and Unity3D
In terms of personal/project planning, I recommend a GTD application. There are quite a few available, and they vary widely in price. I use Things, and have for about a month and a half, and its kept me more on-track than any other method I've used before.
If you've never heard of GTD before, check Amazon/Kindle (the Kindle Reader another good app for the iPhone, although the technicial library is lacking at the time being) for "Getting Things Done."
If you're referring to running apps written in C# on the iPhone, it's very early days, but efforts are underway thanks to the Mono team: